1. Field of the Invention
The present invention relates to a directional coupler for communication equipment.
2. Description of the Related Art
For example, a directional coupler described in Japanese Unexamined Patent Application Publication No. 11-261313 is known. As shown in FIG. 9, the directional coupler described in Japanese Unexamined Patent Application Publication No. 11-261313 includes multiple dielectric layers that are arranged on one another. Electrode patterns are provided on the dielectric layers. The directional coupler includes a first main line 33, a second main line 34, and a first sub-line 35, each of which is composed of a strip line. Both the first main line 33 and the second main line 34 are coupled to the first sub-line 35. The directional coupler is capable of realizing the basic operation in the same manner even if the function of the main lines is exchanged with that of the sub-line because of its structure. The same applies to a technical problem and solutions to the problem described below.
However, since the first main line 33 and the second main line 34 are electromagnetically coupled to a common part of the first sub-line 35 in the directional coupler described in Japanese Unexamined Patent Application Publication No. 11-261313, there is a problem in that the isolation between the first main line 33 and the second main line 34 is poor.